Optical switching device comprising two waveguides whereof both smallest dimensions are less than the guided wavelengths

ABSTRACT

Each guide has an elongated shape, comprises an input at one end and an output at the other end, is arranged on the surface such that all the input ends of all the guides can be simultaneously illuminated in one zone by said radiation to be switched. The invention is characterized in that each guide is produced in a different material; preferably, the guides have more or less the same dimensions, thereby enabling to obtain particularly economical switching means.

This application claims the benefit, under 35 U.S.C. §365 ofInternational Application PCT/FR01/03576, filed Nov. 15, 2001, which waspublished in accordance with PCT Article 21(2) on May 30, 2002 in Frenchand which claims the benefit of French patent application No. 0015228,filed Nov. 24, 2000.

The invention relates to the use of radiation waveguides whose twosmallest dimensions are smaller than the wavelengths of this radiation,which waveguides are capable, however, of guiding this radiation thanksto “plasmon” resonance phenomena.

The article entitled “Plasmon polaritons of metallic nanowires forcontrolling submicron propagation of light”, published on 15 Sep. 1999in the journal Physical Review B, Vol. 60, No. 12, p. 9061-8, written byJean-Claude Weeber et al., describes the principles of selectiveelectromagnetic energy transport using metal “nanowaveguides” depositedon a dielectric substrate.

These elongate metal particles are, for example, deposited on thedielectric substrate using cathode-type lithographic methods.

This article gives examples illustrating these principles, based on theuse, as “nanowaveguides”, of elongate parallelepipeds whose two smallestdimensions, namely the height h and the width e, are smaller than thewavelength of the radiation to be selectively propagated within them:

-   -   by illuminating the end of a gold “nanowaveguide” having        dimensions of 1500 nm×30 nm×15 nm, the normalized light        intensity detected at the other end is a maximum (40%) for a        wavelength of 835 nm and a minimum (zero) for a wavelength of        633 nm;    -   by illuminating the end of a “nanowaveguide”, again made of        gold, having dimensions of 1000 nm×30 nm×20 nm, the normalized        light intensity detected at the other end is a maximum for a        different wavelength, namely 770 nm.

Since the maximum transmission of these “nanowaveguides” depends on thewavelength to be transmitted, we may speak of selective electromagneticenergy transport or of a waveguide with spectral selectivity; the meansused here to obtain this selectivity is based on the difference indimensions, especially the two smallest dimensions, of thenanowaveguides which, in this case, are made in the same material.

That document describes the following example, shown in FIG. 9:

-   -   two identical “nanowaveguides” 1 μm in length are produced using        the same material, are deposited on a dielectric surface and are        placed parallel to each other at a distance of about 0.2 μm        apart, so as to be able to simultaneously excite two        neighbouring ends of these nanowaveguides using the same beam;        these excitation ends are called “entrance” ends;    -   deposited at the other end of each “nanowaveguide” is a gold        “nanoparticle”, one having dimensions of 20 nm×30 nm×100 nm and        the other dimensions of 30 nm×30 nm×100 nm; and    -   the entrance ends of the nanowaveguides are illuminated        simultaneously and it is found that, depending on the        illumination wavelength, one or other of the nanoparticles        scatters radiation.

Thus, this document teaches that a system comprising two nanowaveguidesplaced around the same point of illumination or entrance region andterminating in nanoparticles of different sizes makes it possible toexcite a nearby particle of one or other of the nanowaveguides at thewavelength of the radiation used.

The means used here to excite the particle is based on the difference inparticle size.

However, the construction of such a system remains difficult, especiallybecause it requires the deposition of nanoparticles at the end of thenanowaveguides and the excitation of just a single particle, and alsobecause it requires elements of different sizes to be deposited on thedielectric substrate.

The object of the invention is to avoid this drawback.

For this purpose, the subject of the invention is an optical switchingdevice for switching radiation, comprising a surface provided withradiation waveguides each having an elongate shape, each comprising anentrance at one end and an exit at the other end, the two smallestdimensions of each waveguide being smaller than the wavelengths of thesaid radiation, these waveguides being placed on the surface so that allthe entrance ends can be simultaneously illuminated by the saidradiation to be switched, characterized in that each waveguide differsby the material of which it is made.

The invention may also have one or more of the following features:

-   -   the said material is chosen from the group comprising gold,        silver, aluminium, copper and mixed indium tin oxide;    -   the nature of the material of the waveguides and the dimensions        of the waveguides are adapted in order to obtain, within the        said waveguides, electron plasma resonance for at least one        possible wavelength of the said radiation to be switched;

The excitation wavelength of the resonance corresponds in general toquite a broad range of wavelengths centred on a peak at which theresonance is at maximum; preferably, the resonance wavelengths of thesaid waveguides are all between 350 nm and 1100 nm;

-   -   the waveguides deposited on the same surface have approximately        the same dimensions.

The dimensions are therefore precluded from acting as selectivity meansor as switching means, as in the prior art; because of this additionalfeature, waveguides of different types but of the same size may bedeposited more economically, using the same process with the samesettings; thus, optical switching of the radiation is then achievedusing waveguides of identical size but different in nature, and thisallows an optical switching device to be fabricated inexpensively.

The expression “approximately identical dimensions” is understood tomean the waveguide dimensions that can be achieved using the samedeposition process and the same adjustments; preferably, the said twosmallest dimensions of the waveguides do not exceed 100 nm; typically,the two smallest dimensions of the waveguides are around 40 nm.

The subject of the invention is also an optical system comprising oneentrance and several exits, characterized in that it comprises anoptical switching device according to the invention, the entrance of thesystem being designed to illuminate simultaneously all the entrance endsof the said waveguides, each of the exit ends of the said waveguidesbeing connected to an exit of the system.

The invention applies most particularly to a device for opticallyreading, at at least two different wavelengths, digital data stored on amedium, such as an optical disc, and comprising, for reading at thedifferent wavelengths, a device for optically switching the said variouswavelengths.

Some optical discs for storing digital data have various recordinglayers that can be read simultaneously provided that read laser beams ofdifferent wavelengths are employed, each tailored to one recordinglayer.

On other optical discs for storing digital data, the data is accessibleby luminescence and also requires an optical read device operating atdifferent wavelengths.

Other optical discs, such as for example those in which the data isstored in the form of “plasmons”, require read devices operating at atleast two different wavelengths.

The optical read devices for reading such data media therefore includemeans for switching the various read or luminescence wavelengths; forthis purpose, it is advantageous to use an optical switching device forswitching the data read radiation, comprising a surface provided withradiation waveguides, each having an elongate shape, each comprising anentrance at one end and an exit at the other end, the two smallestdimensions of each waveguide being smaller than the wavelengths of thesaid radiation, these waveguides being placed on the surface so that allthe entrance ends can be simultaneously illuminated in one region by thesaid read radiation.

Preferably, each waveguide differs by the material of which it is made.

Such a switching device is very light and very compact; it can thereforebe very easily integrated into the read head of the read device.

The invention relating to the switching device will be more clearlyunderstood on reading the description that follows, given by way ofnon-limiting example and with reference to the appended figures inwhich:

FIG. 1 shows a diagram of the device according to the invention—theupper part in top view and the lower part in side view; and

FIG. 2 shows the normalized scattered intensity at the exit end of thewaveguides of the device according to the invention as a function of thewavelength.

Using conventional cathode-based lithography methods, waveguides 1, 2 ofidentical dimensions, but differing in type, are deposited on a glasssubstrate 4 having an index of 1.5; the device shown schematically inFIG. 1 is thus obtained.

The first waveguide 1 is made of aluminium and the second waveguide 2 ismade of gold; the common dimensions of the waveguides are 2 000 nm×40nm×40 nm; the waveguides are placed so as to be parallel and at adistance of about 0.1 μm apart, that is to say at a distance largeenough to avoid any direct coupling effect between them but small enoughfor it to be possible for the two neighbouring ends of thesenanowaveguides to be simultaneously excited using the same light beam,these two ends being consequently termed entrance ends and the otherends termed exit ends.

We will now show how the device thus obtained can be used as an opticalswitching means.

As shown in FIG. 1, the beam of the radiation to be switched is made toconverge on an entrance region 3 straddling the entrance ends of the twoguides.

This incident radiation may be focused through the glass substrate by asuitable optical device such as, for example, an immersion objective; itis possible to choose a value close to 1.5 as refractive index of theimmersion oil of this objective; it is possible to choose a value closeto 0.9 as the optical aperture of this objective.

Using suitable computing means known per se, the scattered intensity atthe exit end of the guides under the abovementioned excitationconditions is evaluated; the results were obtained at a distance ofaround 30 nm above the upper surface of the exit end of the guides; theresults are normalized to the incident light and are plotted in FIG.2—the continuous line is for the aluminium waveguide and the broken lineis for the gold waveguide; the peaks in these figures correspond to thewavelengths capable of exciting the electron plasma resonance within thecorresponding waveguide; for example, a maximum resonance is observed at489 nm within the aluminium waveguide, and is observed at 790 nm withinthe gold waveguide.

Under these excitation conditions, by observing the spatial distributionof the electromagnetic field intensity along the waveguides:

-   -   under 489 nm excitation, it is found that the electromagnetic        wave propagates only in the aluminium waveguide;    -   under 694 nm excitation, it is found that the electromagnetic        wave propagates simultaneously in both waveguides; and    -   under 790 nm excitation, it is found that the electromagnetic        wave propagates essentially only in the gold waveguide.

Thus, the system thus obtained can therefore be really used as anoptical switching means.

According to an alternative embodiment of the invention, severalnanowaveguides, of identical dimensions but of different materials, aredeposited on the glass substrate so as to obtain more than two exits,the entrance being still shared between the various nanowaveguides;according to this embodiment, the nanowaveguides are arranged, forexample, as a star around the entrance or excitation region on which theincident beam converges; these various materials will be chosen in amanner known per se so that the wavelengths for maximum resonance do notoverlap.

The optical switching device according to the invention is lessexpensive than those of the prior art, especially because all thenanowaveguides of this device have the same size and can be producedusing the same deposition process with the same settings.

The optical switching device according to the invention mayadvantageously be integrated into an optical read device for reading adigital data storage medium.

1. Optical switching device for switching radiation, comprising asurface provided with radiation waveguides each having an elongateshape, each comprising an entrance at one end and an exit at the otherend, the two smallest dimensions of each waveguide being smaller thanthe wavelengths of said radiation, these waveguides being placed on thesurface so that all the entrance ends can be simultaneously illuminatedin a region by said radiation to be switched, wherein each waveguidediffers by the material of which it is made.
 2. Optical switching deviceaccording to claim 1, wherein the nature of the material of thewaveguides and the dimensions of the waveguides are adapted in order toobtain, within said waveguide, electron plasma resonance for at leastone possible wavelength of said radiation to be switched.
 3. Opticalswitching device according to claim 2, wherein the resonance wavelengthsof said waveguide are all between 350 nm and 1100 nm.
 4. Opticalswitching device according to claim 1, wherein said material is chosenfrom the group comprising gold, silver, aluminium, copper and mixedindium tin oxide.
 5. Optical switching device according to claim 1,wherein the waveguides deposited on the same surface have approximatelythe same dimensions.
 6. Optical switching device according to claim 1,wherein said two smallest dimensions of the waveguides do not exceed 100nm.
 7. Optical system comprising one entrance and several exits, whereinit comprises an optical switching device according to claim 1, theentrance of the system being designed to illuminate simultaneously allthe entrance ends of said waveguide, each of the exit ends of the saidwaveguides being connected to an exit of the system.
 8. Device foroptically reading, at at least two different wavelengths, digital datastored on a medium, such as an optical disc, and comprising, for readingat the different wavelengths, an optical switching device for switchingthe data read radiation, comprising a surface provided with radiationwaveguides, each having an elongate shape, each comprising an entranceat one end and an exit at the other end, the two smallest dimensions ofeach waveguide being smaller than the wavelengths of said radiation,these waveguides being placed on the surface so that all the entranceends can be simultaneously illuminated in one region by said readradiation.
 9. Device according to claim 8, wherein each waveguidediffers by the material of which it is made.